按Java8降序对地图进行排序 [英] Sort map in descending order java8
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问题描述
private static <K, V extends Comparable<? super V>> Map<K, V>
sortByValue( Map<K, V> map )
{
Map<K, V> result = new LinkedHashMap<>();
Stream<Map.Entry<K, V>> st = map.entrySet().stream();
st.sorted( Map.Entry.comparingByValue() )
.forEachOrdered( e -> result.put(e.getKey(), e.getValue()) );
return result;
}
This is an example from this post. It works. the problem is that it sorts in ascending order. How can I change it to descending ?
我可以这样:
public static <K, V extends Comparable<? super V>> Map<K, V>
sortByValue( Map<K, V> map )
{
List<Map.Entry<K, V>> list =
new LinkedList<Map.Entry<K, V>>( map.entrySet() );
Collections.sort( list, new Comparator<Map.Entry<K, V>>()
{
public int compare( Map.Entry<K, V> o1, Map.Entry<K, V> o2 )
{
return (o2.getValue()).compareTo( o1.getValue() );//change o1 with o2
}
} );
Map<K, V> result = new LinkedHashMap<K, V>();
for (Map.Entry<K, V> entry : list)
{
result.put( entry.getKey(), entry.getValue() );
}
return result;
}
我可以通过在以下行中更改顺序来做到这一点:return (o2.getValue()).compareTo( o1.getValue() );
但我想尝试使用lambda expr.
I can do that by changing order i this line: return (o2.getValue()).compareTo( o1.getValue() );
But I would like to try with lambda expr.
推荐答案
You can use Comparator
's default method reversed()
to reverse the sense of comparisons to sort it descending.
这里的类型推断似乎有点不足,但是向comparingByValue()
提供显式的类型参数可以解决此问题.
The type inference seems to be a little off here, but supplying explicit type arguments to comparingByValue()
fixes the issue.
st.sorted( Map.Entry.<K, V>comparingByValue().reversed() )
.forEachOrdered(e -> result.put(e.getKey(), e.getValue()));
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